Pressure transducers are known to take a variety of forms. One form relies on the pressure-induced deflection of a thin diaphragm. In the case of a capacitive pressure transducer, the diaphragm deflection causes a variation in the distance between a pair of surfaces which form the plates of a variable capacitor. In U.S. Pat. No. 3,634,727, there is disclosed a capacitor formed of two wafers of silicon insulated from each other by glass and joined together by a low temperature glass or by brazing thin metal films deposited on the glass. The device disclosed in said patent requires the processing of two wafers of silicon to provide only a single transducer, and the method of joining the two wafers of silicon is cumbersome. In order to process wafers to provide a large number of pressure transducers from a single pair of processed wafers, a suitable joining technique must be utilized. One such is the use of field-assisted bonding in which a layer of borosilicate glass between the two pieces of silicon permits bonding of the silicon with the borosilicate glass at about 500.degree. C. in a vacuum, the silicon pieces being totally attracted to one another by a DC electric field established by on the order of 100 or 200 volts impressed between them, described in NASA Tech Brief B74-10263, January 1975, entitled "Low-Temperature Electrostatic Silicon-To-Silicon Seals Using Sputtered Borosilicate Glass". The device in the aforementioned patent also suffers from a very low ratio of variable capacitance to parasitic fixed capacitance inasmuch as the periphery of the device has conductive closer to each other than the surfaces of the deflectable portion of the device.
In order to improve the variable to fixed capacitance ratio, and particularly to mitigate the parasitic fixed capacitance (that part which is not varied as a function of diaphragm flexure in response to pressure changes), it is necessary to provide topographical shaping, such as moats, pedestals or pistons, to cause the relatively movable capacitive plate portions to be close to each other in contrast with the fixed portions of the conductive body of the device. Additionally, in the event that very small devices are made (such as by large scale integrated circuit processing of wafers to form a plurality of devices per wafer pair), the small area of the opposed capacitive plates requires close spacing in order to have an adequate capacitance for desired sensitivity to pressure.
In mass production of silicon capacitive pressure transducers utilizing known microcircuit and thin film technology, particularly where small devices are desired and capacitor plate surface spacing becomes very small, it is necessary that the processes be selected and performed in such a fashion as to control dimensions very accurately. If, for instance, a pedestal or piston is too tall, short circuiting can result in a bad device; is a pedestal or piston is too short, then an inadequate capacity or variable capacitance as a function of pressure may result.